In conventional low-power voltage reference generators, when a target output voltage target is raised, output current must be increased to reach the target output voltage. Raising output current increases power consumption and generates excessive heat.
FIG. 1 depicts a conventional low-power voltage reference generator 100 that is a weak inversion CMOS voltage reference generator. A biasing current IB is generated and controlled by a self-stabilizing reference voltage circuit containing transistors Q1, Q2, Q3, and Q4, each of which function in saturated weak inversion. The biasing current IB varies non-linearly, and has a positive temperature coefficient. In other words, the biasing current IB increases with an increase in temperature. Through the self-stabilization feature of this circuit, the biasing current IB generates a control voltage that is input to transistors Q3 and Q5.
Transistors Q5 and Q6, as well as a resistor R, form a current mirror that mirrors the biasing current IB. The voltage drop across resistor R and transistor Q6 caused by flow of the mirrored biasing current IB generates the voltage reference generator's 100 output voltage (Vo). The transistor Q5 operates under weak inversion in a sub-threshold region. Transistor Q6 is configured as a diode, thus the voltage drop across the transistor Q6 varies non-linearly, with a negative temperature coefficient. That is, the voltage drop across transistor Q6 decreases with an increase in temperature. The combined effects of temperature changes on the biasing current IB and the voltage drop across transistor Q6 tend to mitigate each other.
However, conventional voltage reference generators, such as the voltage reference generator 100, provide an output voltage that is too low for some applications. A conventional approach toward this problem includes increasing current flow through the device. Another conventional response to this problem uses bipolar transistors, which require excessive space. Accordingly, classic techniques for raising the output voltage increase power consumption, excessively increase temperature effects, and/or significantly increase circuit size.
Thus, there are long-felt industry needs for methods and apparatus that mitigate the shortcomings of conventional methods and apparatus, including a low-power voltage reference circuit that can achieve an increase in target output voltage over conventional devices without a commensurate increase in operating current and/or an excessive increase in circuit size.